Multi-carrier technology has been proposed in order to enhance the spectrum efficiency of a light transmitting system. Up to date, this technology mainly uses WDM, OFDM and Superchannel proposed by Bell Laboratory in 2010.
Among them, WDM is the English abbreviation for wavelength division multiplexing, in which the carrier separation is generally on the order of tens GHz. OFDM is the English abbreviation for orthogonal frequency division multiplexing, and was introduced into the optical communication field from the wireless communication field. Up to date, OFDM may be divided into traditional OFDM, in which OFDM signals generated in electrical domain are loaded onto optical carriers and thus the sub-carrier separation is small and usually on the order of MHz, and all-optical OFDM, in which signals are generated, similar to WDM, fully in optical domain, but the sub-carrier intervals are smaller and the number of carriers is comparatively small. Superchannel is based on a plurality of phase-locked sub-carriers generated by one light source, with the purpose of greatly increasing spectrum efficiency of the system.
However, multi-carrier optical communication systems reported up to now are mainly based on Phase Shift Keying (PSK) modulation technique or Quadrature Amplitude Modulation (QAM) technique, not involving a differential encoding modulation technique, such as Differential Amplitude Phase Shift Keying (DAPSK) modulation technique. Both PSK and QAM encoded multi-carrier systems have problems with inter-carrier interference and various corresponding methods are available for compensation for a specific system.
In an optical OFDM system which is a present research hotspot, since OFDM signals obtained by using a conventional modulation technique have a higher peak-power ratio, effects caused by optical fiber nonlinearity become a tough problem. As for conventional PSK or QAM encoded OFDM signals, known data such as training symbols, pilot sub-carriers or a separately inserted radio frequency signal is used to perform channel estimation, phase noise compensation and so on, which on one hand increases data redundancy and reduces spectrum efficiency, on the other hand increases the complexity of the digital signal processing at the receiver.